NAME
rand, srand,
rand_r —
bad random number generator
LIBRARY
library “libc”
SYNOPSIS
#include
<stdlib.h>
void
srand(unsigned
seed);
int
rand(void);
int
rand_r(unsigned
*ctx);
DESCRIPTION
The
rand()
function computes a sequence of pseudo-random integers in the range of 0 to
RAND_MAX, inclusive.
The
srand()
function seeds the algorithm with the seed parameter.
Repeatable sequences of rand() output may be
obtained by calling srand() with the same
seed. rand() is implicitly
initialized as if srand(1) had
been invoked explicitly.
In FreeBSD 13,
rand() is
implemented using the same 128-byte state LFSR generator algorithm as
random(3). However, the legacy
rand_r()
function is not (and can not be, because of its limited
*ctx size). rand_r()
implements the historical, poor-quality Park-Miller 32-bit LCG and should
not be used in new designs.
IMPLEMENTATION NOTES
Since FreeBSD 13,
rand() is implemented with the same generator as
random(3), so the low-order bits should no longer be significantly
worse than the high-order bits.
SEE ALSO
STANDARDS
The rand() and
srand() functions conform to
ISO/IEC 9899:1990
(“ISO C90”).
The rand_r() function is not part of
ISO/IEC 9899:1990 (“ISO C90”)
and is marked obsolescent in IEEE Std 1003.1-2008
(“POSIX.1”). It may be removed in a future revision of
POSIX.
CAVEATS
Prior to FreeBSD 13,
rand() used the historical Park-Miller generator
with 32 bits of state and produced poor quality output, especially in the
lower bits. rand() in earlier versions of
FreeBSD, as well as other standards-conforming
implementations, may continue to produce poor quality output.
These functions should not be used in portable applications that want a high quality or high performance pseudorandom number generator. One possible replacement, random(3), is portable to Linux — but it is not especially fast, nor standardized.
If broader portability or better performance is desired, any of the widely available and permissively licensed SFC64/32, JSF64/32, PCG64/32, or SplitMix64 algorithm implementations may be embedded in your application. These algorithms have the benefit of requiring less space than random(3) and being quite fast (in header inline implementations).